From a chemical point of view, woods are constituted of four major components: cellulose, hemicellulose, lignin and extractives. In order to make cellulosic pulp from wood products, the wood fibers, and particularly their main constituent cellulose, must be liberated from the other components. It is in the digestor section of pulping processes that fiber liberation by delignification is achieved. However, it is also important that the delignification be conducted under conditions which do not deleteriously affect fiber quality. It is the objective of wood pulping, or digestion, to separate the cellulose fibers one from another in a manner that preserves the inherent fiber strength and to remove as much of the lignin, extractives and hemicellulose materials as is required by end-use considerations. While a number of pulping processes are known, three principal chemical pulping processes are the soda, the kraft, and the sulfite processes.
The soda process uses sodium hydroxide as the cooking chemical for delignification purposes, and has been largely superseded by the kraft process.
Kraft processes are applicable to nearly all species of wood and are characterized by their use of sodium hydroxide and sodium sulfide as the active delignification agents in the digestor. During this treatment, lignin is extensively degraded and the degradation products are dissolved. Carbohydrates, in particular hemicelluloses, undergo partial degradation and dissolution. Extractives are, to a large extent, removed.
In contrast to the kraft processes, sulfite pulping processes are sometimes used. The sulfite processes utilize calcium, sodium, magnesium, or ammonium bisulfite in combination with free or excess sulfur dioxide as the cooking chemicals in the digestor. Bisulfite processes use sodium, magnesium, or ammonium bisulfite in the digestor.
Without doubt, the various kraft processes are most frequently used for papermaking today.
However, the kraft process involves relatively complicated capital and energy intensive recovery cycles for recycling the cooking chemicals back to the digestor section. Thus, notwithstanding the virtually universal acceptance of kraft or alkaline processes for pulping wood and papermaking processes, current kraft pulping processes are characterized by prolonged impregnation and digestion times due to mass and heat transfer limitations, complicated recovery cycles, and non-uniform pulp quality. Furthermore, delignification is relatively incomplete in the digestor and post digestor delignification is frequently necessary.
The pulping processes which yield flexible fibers without undue carbohydrate damage produce papers of the highest strength. Flexible fibers produce paper with a relatively large area of fiber-to-fiber contact, resulting in sheets of higher strength. The amount of lignin left in the pulp has a bearing on the tear, burst, and fold properties of paper. Because these properties increase with decreasing lignin content it is desirable to remove as much lignin as reasonable costs permit. It is, therefore, clear that there exists a great need in the art for improved efficiency in the digestion of wood so as not only to maintain or improve delignification, but also to reduce the cost of further pulp processing.